Ceramic dielectric resonators are used as bandpass or bandstop filters and oscillator stabilizer devices in the microwave frequency range of 1 to 12 GHz. These devices are becoming increasingly important with the continued development of microwave integrated circuitry, microwave telecommunications, and satellite broadcasting systems. See for example: (1) S. B. Cohn, IEEE Transactions on Microwave Theory and Technology MTT-16 (4), 218-227 (1968); (2) J. K. Plourde and C. L. Ren, IEEE Transactions on Microwave Theory and Technology MTT-29 (8), 754-770 (1981); (3) S. Nomura, Ferroelectrics 49 (1/2/3/4), 61-70 (1983); (4) H. Ouchi and S. Kawashima, Japanese Journal of Applied Physics 24 (Supplement 24-2), 60-64 (1985); and (5) K. Wakino, Proceedings of the Sixth IEEE International Symposium on Applications of Ferroelectrics, 97-106 (1986).
Although each application has specific requirements, important properties of microwave dielectric properties for resonator applications are the following:
The operating frequency, f, of a dielectric resonator is in the microwave frequency range of 1 to 12 GHz. PA1 The relative dielectric permittivity or dielectric constant, should be greater than 30, and preferably larger than 80, depending on the specific application. PA1 The dielectric loss or dissipation factor, tan .delta., should be less than 0.001, and preferably less than 0.0002 at the operating frequency. PA1 The dielectric loss quality, Q, which is approximately equal to 1/tan .delta., should be greater than 1000, and preferably greater than 5000 at the operating frequency. PA1 The temperature coefficient of resonant frequency, T.sub.f, which is derived from the combined effects of thermal expansion and the temperature dependence of the dielectric permittivity, T.sub. , should be less than 10 ppm/.degree.C., and preferably zero. PA1 (a) substitutions for Bi selected from the group consisting of Ca, Sr, Ba, Y, Pb, Cd, La and other rare earth elements having atomic numbers 58-71 of the periodic table, wherein said substitutions comprise less than 20 mol % based on Bi content; (b) substitutions for Zn selected from the group consisting of Mg, Ca, Co, Mn, Ni, and Cu, wherein said substitutions comprise less than 20 mol % based on Zn content; and (c) substitutions for Nb selected from the group consisting of Sn, Ti, Zr, Hf, and Ta, wherein said substitutions comprise less than 20 mol % based on Nb content; however, more preferred are substitutions for Bi, Zn, or Nb that are each less than 10 mol % based on Bi, Zn, or Nb content respectively. The above compositions typically have properties where: PA1 more preferably the characteristics comprise: PA1 (b) substitutions for Zn selected from the group consisting of Mg, Ca, Co, Mn, Ni, and Cu, wherein said substitutions comprise less than 20 mol % based on Zn content; and (c) substitutions for Nb selected from the group consisting of Sn, Ti, Zr, Hf, and Ta, wherein said substitutions comprise less than 20 mol % based on Nb content; however, more preferred are substitutions for Bi, Zn, or Nb that are each less than 10 mol % based on Bi, Zn, or Nb content respectively. The compositions typically have properties where: PA1 more preferably the characteristics comprise: PA1 Dielectric constant: .sub.r =80 to 145 PA1 Temperature dependence of : T.sub. =-360 to 200 ppm/.degree.C. PA1 Temperature dependence of resonant frequency: .vertline.T.sub.f .vertline..ltoreq.200 ppm/.degree.C. PA1 Tan .delta. at 100 kHz: tan .delta.&lt;0.0002 PA1 Product of Microwave Q.multidot.f, (1-5 GHz): Q.multidot.f&gt;1000 GHz; PA1 Dielectric constant: .sub.r =90 to 110 PA1 Temperature dependence of resonant frequency: .vertline.T.sub.f .vertline..ltoreq.20 ppm/.degree.C. PA1 Product of Microwave Q.multidot.f, (1-5 GHz): Q.multidot.f&lt;5000 GHz. PA1 Dielectric constant: .sub.r =80 to 145 PA1 Temperature dependence of : T.sub. =-360 to 200 ppm/.degree.C. PA1 Temperature dependence of resonant frequency: .vertline.T.sub.f .vertline..ltoreq.200 ppm/.degree.C. PA1 Tan .delta. at 100 kHz: tan .delta.&lt;0.0002 PA1 Product of Microwave Q.multidot.f, (1-5 GHz): Q.multidot.f&gt;1000 GHz; PA1 Dielectric constant: .sub.r =90 to 110 PA1 Temperature dependence of resonant frequency: .vertline.T.sub.f .vertline..ltoreq.20 ppm/.degree.C. PA1 Product of Microwave Q.multidot.f, (1-5 GHz): Q.multidot.f.gtoreq.5000 GHz.
The scope of the present invention involves the use of dielectric resonant filters in microwave applications such as mobile telephones. These applications require ceramic materials with .sub.r &gt;80, Q&gt;1000, and .vertline.T.sub.f .vertline.&lt;10 ppm/.degree.C., within the desired frequency range of 1-2 GHz. It is difficult to achieve this combination of properties in present ceramic materials.
For the purposes of comparison of different microwave dielectric ceramics, where measurements are often made at different frequencies, it is useful to consider the theoretically predicted and experimentally confirmed relationship that the Q.multidot.f product is constant over the microwave frequency region of 1 to 12 GHz. See for example: (6) K. Wakino, Ferroelectrics 91, 69-86 (1989).
Microwave Dielectric Ceramic Compositions
A family of dielectric ceramics have been developed with compositions in the BaO--RE.sub.2 O.sub.3 --TiO.sub.2 system, where RE.sub.2 O.sub.3 is a rare earth oxide such as Nd.sub.2 O.sub.3, Sm.sub.2 O.sub.3, or La.sub.2 O.sub.3, with additions of PbO, SrO, or Bi.sub.2 O.sub.3. See for example: reference (4) cited above; (7) K. Wakino, et al., Journal of the American Ceramic Society 67 (4), 278-281 (1984); (8) S. Nishigaki, et al., American Ceramic Society Bulletin 66 (9), 1405-1410 (1987); (9) K. Kageyama and M. Takata, Japanese Journal of Applied Physics 24 (Supplement 24-2), 1045-1047 (1985); and (10) J. M. Durand and J. P. Boilot, Journal of Materials Science Letters 6, 134-136 (1987). Dielectric ceramics of this family have relatively high permittivity ( .sub.r =70-80), very low temperature dependence (.vertline.T.sub.f .vertline.&lt;5 ppm/.degree.C.), and extremely low microwave loss (Q.multidot.f=5000 to 10,000 GHz).
Bi.sub.2 O.sub.3 --ZnO--Nb.sub.2 O.sub.5 Ceramics
Promising results have been obtained for dielectric ceramics in the Bi.sub.2 O.sub.3 --ZnO--Nb.sub.2 O.sub.5 (BZN) ceramic system, with various compositional modifications (e.g., NiO, CaO). These ceramics also have the advantage of relatively low sintering temperatures (less than 1000.degree. C.). BZN ceramics have been developed as dielectric materials for multilayer capacitors, which operate at low frequencies (1 kHz to 1 MHz). See for example: (11) Wang Zhenping and Zhang Shiying, Proceedings of the 37th Electronic Components Conference (Catalog No. 87CH2448-9), 413-419 (1987). These workers identified dielectric ceramics with molar compositions of Bi.sub.2 (Zn.sub.x Nb.sub.1.50)O.sub.6.75+x (where x=0.8 to 1.0) and Bi.sub.2 Zn.sub.0.8 Nb.sub.x O.sub.3.8+2.5 x (where x=0.70 to 0.82). These BZN ceramics exhibited the following dielectric properties: .sub.r =75 to 140, tan .delta.&lt;0.0005, and T.sub. =-75 to +60 ppm/.degree.C. Further improvement in dielectric properties were achieved in BZN ceramics by additions of Bi.sub.2 (Ni.sub.1.33 Nb.sub.0.67)O.sub.6.67. Results of dielectric measurements at microwave frequencies were not provided.
Low-frequency dielectric properties of BZN ceramics with compositions represented by the formula, Bi.sub.2 (ZnNb.sub.2(1+d))O.sub.3+6y+5dy (where 0.6&lt;y&lt;1.0 and -0.05&lt;d&lt;0.05) also have been reported. See for example: (12) M. F. Yan, et al., Journal of the American Ceramic Society 73 (4), 1106-1107 (1990); (13) H. C. Ling et al., Journal of Materials Research, 5 (8), 1752-1762 (1990); and (14) H. C. Ling and M. F. Yan, U.S. Pat. No. 4,638,401 (1987). Temperature-stable dielectric ceramics with .sub.r values between 80 and 90, and tan .delta. values between 0.0003 and 0.0005 at 1 MHz, were obtained in this system. Properties of these BZN ceramics were further improved by additions of Bi.sub.3 Ni.sub.2 NbO.sub.9. Results of dielectric measurements at microwave frequencies were not provided.
The microwave dielectric properties of a range of compositions in the BZN system recently have been reported. See for example: (15) H. Kagata, et al., Japanese Journal of Applied Physics 31, 3152-3155 (1992); and (16) H. Kagata and J. Kato, Japanese Patent Application No. 4-285046 (1992). Molar compositions evaluated were represented by the formula, xBiO.sub.1.5.yZnO.zNbO.sub.2.05 (where 0.41&lt;x&lt;0.51, 0.19&lt;y&lt;0.30, and 0.25&lt;z&lt;0.345). BZN ceramics within this range exhibited the following microwave dielectric properties: .sub.r =89 to 133, Q=40 to 310 at 2-4 GHz, and T.sub.f =-110 to +120 ppm/.degree.C. It was also shown that a BZN ceramic with the composition Bi.sub.18 Zn.sub.8 Nb.sub.12 O.sub.65 had microwave dielectric properties of: .sub.r =82, Q=300 at 3.2 GHz, and T.sub.f =-100 ppm/.degree.C. Refinement of the BZN composition with CaO substitution for ZnO resulted to a composition consisting of 45.75BiO.sub.1.5 21.75(Ca.sub.0.725 Zn.sub.0.275) )O.cndot.32.5NbO.sub.2.5, with .sub.r =79, Q=360 at 3.2 GHz, and T.sub.f =+1 ppm/.degree.C.